Method of generating combustion gases by burning a gaseous combustible mixture



Apnl 1, 1958 e. DOBSON 2,828,605

METHOD OF GENERATING COMBUSTION GASES BY BURNING A GASEOUS COMBUSTIBLE MIXTURE Filed March 17. 1952 s Sheets-Sheet 1 FUEL. n. SouQcE.

HEAT EXCHAMGEQ.

UR E 0: LOW CALOQIC.

,0, Attorne United Stats METHOD 9F GENERATING COMBUSTION GASES BY BURNENG A GASE'GUS COMBUSTIBLE MIX- TURE George Dohson, East Sheen, London, England, assignor to Power lets (Research and Development) Limited, London, England, a company of Great Britain This invention relates to the generation of combustion gases for use in the prime mover of thermal power plant, particularly gas turbine engine plant, by burning a low calorific value fuel, for example, methane and air mixtures of low concentration of methane, such as are obtainable from the ventilated or upcast air from certain collieries, which cannot be burnt at normal temperatures and pressures, and which usually are wasted. Another suitable fuel would be the gases resulting from the underground gasification of coal. I

According to the invention a gas-turbine engine plant designed to be operated by a gaseous fuel and air mixture of low calorific value comprises at least one combustion chamber, a flame tube therein and a burner for burning an auxiliary supply of fuel, and the tube is provided with inlets for the admission of the gaseous fuels and air mixture containing excess of air in successive stages along its length, the arrangement being such that low calorific fuel can be admitted to the combustion chamber, heated by the burner and burnt, further quantities of such fuel and air being admitted subsequently ati spaced intervals along the flame tube, so that the temperature of the accumulating gaseous combustion products gradually decreases until the temperature of the products is too low to ignite any further addition of the low calorific fuel, whereupon if necessary, afinal addition of cooling gas is added to reduce the temperature of the gaseous products to that required before entry to the gas-turbine.

One construction and operation of apparatus according to the invention is described by way of example with reference to the accompanying drawings of which Figure 1 shows the general arrangement of a gas-turbine plant, Figure 2 is a longitudinal section through the combustion chamber and Figure 3 is a cross-section on the line 3--3 of Figure 2. Figures 4, and 6 are graphs illustrating the operation.

In Figure 1 the installation comprises the conduit 1 leading from the source 2 of low-calorific-value gaseous fuel to the axial-flow compressor 3. The latter is connected by' conduit 4 to the heat-exchanger 5 which is connected by conduit 6 to the combustion chamber 7. The outlet of the chamber 7 is connected by the conduit 8 to the turbine 9 which, in addition to driving the load 22, also drives the compressor 3. The turbine discharges through the heat-exchanger 5 to the exhaust 10. The fuel from source 2 is a very weak mixture of combustible gas-e. g. methane-with air.

An auxiliary supply of fuel, which may be gas or powdered coal but which in this example is assumed to be oil, is fed to the combustion chamber 7 through pipe '11 by pump 12 from reservoir 13.

The combustion chamber as seen in Figures 2 and 3 consists of the cylindrical chamber 14 and an annular flame tube inside made up of the outer tube 15 and the inner tube 16. At its inlet end the chamber 14 diverges in usual manner, to form a diffuser for the incoming gas;

5 3 get the inner tube 16 and outer tube 15 similarly diverge. At the outlet end the flame tube merges into a cylindrical tube at and is attached to the wall of chamber 14 adjacent to the attachment of conduit 8.

Projecting through the end of the annular tube is a ring of oil burners 17 connected to the tube by the usual ring of swirl vanes 18 around each burner serving in Well-known manner to admit the air mixture in a vortex flow which sets up a stable zone of combustion into which the fuel oil is injected. The burners are connected to common oil pipe 19 which passes out through the wall of chamber 14 for connection to the pipe 11. At each of a number of positions along its length the flame tube is provided with rings of ports 15a, 15b 15m, 15m formed by perforating both the tubes 15 and 16. Thus in addition to the gaseous fuel-air mixture admitted through the swirl vanes 18, further additions of fuel-air mixture are admitted through the ports along the tube at spaced intervals. The spacing between 15a, 15b etc. is such that suflicient time is allowed for mixing of the fuel admitted and the combustion products already formed and then complete combustion, before fresh gas and air mixture is added. The sizes of the admission ports, and the respective combustion zone lengths between them and the number of sets of ports, are determined by the kind of inflammable gas, the concentration of the gas in the air, and the temperature or velocity of the gas in the flame tube.

The auxiliary fuel in relatively small quantity is thus burnt as a pilot-flame at an initial zone at one end of the flame tube and a quantity of weak gas-air mixture, under pressure from the compressor 3 and heated by the heat exchanger 5, is admitted subsequently into the first mixing zone between ports 15a and 15b, heated and burnt,

more gas-air mixture'being admitted subsequently by the appropriate ports into the next mixing zone downstream of ports 15b, mixed with the combustion products from the previous combustion stage, and burnt; the addition of further supplies of gas-air mixtures, the mixing and the burning are continued until eventually the temperature of the products at some stage inthe combustion space becomes lowered so that it is below the ignition temperature of the fuel-air mixture but may still be above the temperature required for the gas which is to be supplied to the gas-turbine. Further quantities of the methane and air mixture are then added, for example through ports 21, or 15n and 21, as cooling medium to reduce the temperature of the gas mixture to the value required for the turbine inlet. Therefore depending on the mixture ignition temperature (which can be determined by a simple test), a certain proportion of the gaseous fuel, is burnt while some is wasted as coolant, but the auxiliary fuel provides the necessary initial high temperature to start the combustion of the weak mixtures. The higher the turbine inlet temperature, and the lower the temperature of the mixture used in the final. cooling to the turbine inlet temperature, the less the wastage of gaseous fuel.

The operation of the system may be illustrated by the following examples in which the pilot flame operates by burning methane and the weak methane-air mixture to be used contains say 4 percent methane and the velocity of the gases through the flame tube is say 50 feet per sec.

Example 1 In this case the inlet temperature of the methane-air mixture is 670 C. and the arbitrary minimum temperature to which the combustion gases may drop when dilut- Referring now to graph shown in Figure 4, the left hand ordinate represents temperature in C., the right hand ordinate represents percentage of the total quantity of methane-air mixture introduced through each set of ports and the abscissa represents the stages of dilution through the successive sets of ports 15a, 15b 1512, the origin being the primary gas inlet around the burners. The points on curve A are the final temperatures of the combustion gases immediately before dilution at each'set of ports, the horizontal line B is the dilution temperature as hereinbefore defined and the points on curve C represent the percentage of mixture used to dilute the combustion gases at each stage. Approximately 16% of the mixture is admitted through the primary inlet and the oil from the burners burns in this gas to give a final temperature at the first set of ports 15a of 1100 C.-

which is represented by the extreme left hand point on curve A. A further 13% of the mixture is then admitted through ports .lSawhich cools gas to the dilution temperature of 900 C. Further combustion takes place in the space between the ports 15a and 15b and the final temperature before dilution at ports 15!) is approximately 1015 C. as shown by the second point on curve A. A further 15% of the mixture is admitted through ports 15b and a similar action takes place. Further quantities of the mixture are added through the subsequent sets of ports until the final gas temperature falls to 950 C., at which temperature combustion of the methane-air mixture can no longer occur. If any further mixture is added (that is through ports 21), it merely serves to cool the combustion gases.

Example 2 In this example, the conditions are similar to those of Example 1, but a lower dilution temperature of 875 C. is accepted for the later stages of dilution. The curves D, E and F of Figure have the same significance as the curves A, B and C respectively of Figure 4. It will be seen that only three stages of dilution are provided, the temperature being allowed to fall to 875 C. at the third stage. The overall combustion efliciency will be reduced to 97% and larger quantities of the mixture are introduced at each stage. Again the end temperature of the gases is approximately 950 C.

Example 3 This case difiers from the previous examples in that a higher dilution temperature is provided for in the early stages of dilution. It will be seen from the curve H that the dilution temperatures for the first two stages are 1000 C. and 950 C. respectively and the dilution temperature is only allowed to fall to the minimum of 900 C. at the third stage. Hence smaller quantities of mixture are required in the early stages. This method again gives a combustion efliciency of 100%.

I claim:

1. The method of generating combustion gases for use in the prime mover of thermal power plant by burning a low calorific value gaseous combustible mixture which comprises the steps of igniting a small portion of the mixture by a pilot flame and causing the burning mixture to flow in a stream, and of adding a succession of further predetermined portions of the mixture to the stream at the successively downstream points at which the preceding portion has become substantially completely burnt, so that the said added further portion is ignited by the heat of the flame resulting from the combustion of the preceding portion.

2. The method according to claim 1 which includes the further step of adding more of the mixture to the stream as coolant at a dowstream point where the temperature has become too low to ignite the further addition.

3. The method of generating combustion gases for use in the prime mover of thermal power plant by burning a gaseous combustible mixture of low calorific value which comprises causing the mixture to flow in a stream to which portions of the mixture are added at a number of successive points, going downstream, the initial portion of the mixture being ignited by a pilot flame and thereby brought to a temperature not below a predetermined value necessary for efficient combustion, and the quantities successively admitted to the stream and the spacing along the stream of the points of addition being such that each portion is ignited by the heat of the stream to which it is admitted and the temperature along the stream up to the last of said points of addition nowhere falls below the predetermined value.

4. The method of generating a stream of combustion gases for use in the prime mover of thermal power plant by burning a gaseous combustible mixture of low calorific value which comprises the steps of first igniting an initial portion of the mixture by a pilot flame at a temperature not below a predetermined value necessary for efiicient combustion, of secondly allowing this ignited portion to flow and burn until most of it has been burnt, of then thirdly adding to the stream a further portion of the mixture, such that the added quantity is insuflicient to reduce the temperature of the stream below the said value and the added portion is ignited by the heat of the stream to which it is admitted, and of repeating the step thirdly recited at least at one further point downstream at which most of the combustible content of the stream has become burnt.

References Cited in the file of this patent UNITED STATES PATENTS 

